It often happens that we repair or simply test guitar pedal circuits and pedals. Here we use one of the most sensitive natural tools. This is the human ear.
A quick and easy-to-build test tool can greatly facilitate repair work. This is the audio signal injection and signal tracking circuits. There are actually two circuits here. A simple astable 555 oscillator and a 386 mini amplifier.
If you connect the audio signal injector to the input of the circuit under test, you can hear and follow the path of the signal. This way, from one end to the other, segment by segment or component by component, you can test where the signal gets stuck. Let’s look at a specific example. Let’s say that a guitar effect doesn’t work, or doesn’t work as it should.Connect the injector to the input and touch the components backwards from the output connector. Until there is a clear, clearly audible signal. For example, for the popular TS808:
Unfortunately, the error can be many things: bad soldering, an improperly connected transistor or fet, an electrolytic capacitor with incorrect polarity, a short circuit, a wiring error, etc etc. Unfortunately, several things. ( Has occured, for example, that a completely new capacitor, tested in principle, was faulty and caused unpleasant moments )
The signal injection unit
The basic test signal injector unit:
And the extended unit:
This is in two parts. A simple 555 astable multivibrator (555, C2,C3,R1,R2) and an RC network (R3,R4,R5,C5,C6,C7) that converts the square wave signal into a sine signal. The circuit emits a 330hz sine signal and this corresponds almost exactly to the E tone of a standard-tuned guitar. So maybe we can call it “guitar E sound simulator”. By modifying the parts, we can produce a different frequency.
A great calculator is here for example: Astabel 555 calculator
But the RC network, which is precisely tuned to the square wave, must also be modified. So if, for example, you change the 555 oscillator to a frequency of 1khz, then the RC network must also be sized to a resonance frequency of 1khz. Otherwise, it does not convert it to a sine wave, it remains a square signal.
And why do we convert the square wave into a sine signal? Well, it’s not necessary, but there are some circuits or devices that “don’t like” when the input signal is a square wave. and the sine signal sounds a little better if it takes a long time to find the error, it doesn’t hurt the ear as much.
The finished circuit:
You can see some differences compared to the schematic diagram: two filter capacitors for the power supply and a potentiometer (R5) for precise frequency adjustment. The jack plug is soldered directly to the prototype panel.
The tracer unit
This is a completely average LM386 mini amplifier, no explanation needed. Connect an DMM instrument cable to the input:
Circuits built on a prototype board. ok, not the prettiest prototype panels in the universe, but it serves the purpose perfectly. 🤷♂️
Useful links: Sine Converter and RC Filter
555 Triangle/Sawtooth Wave Generator
Some caveats:
It is important that you know the circuit and how it works.
Also, NEVER USE it to test a mains circuit or a circuit operating at high voltage (e.g. electron tube).
Test only battery powered circuits. Max 9V
Do not use with headphones
All responsibility is yours.
*Errors may occur.